854 Mr. H. E. Ives on the 



Photometric Conditions. 



Flicker Photometer. — The flicker photometer was chosen 

 for reasons made clear in the previous papers of this series. 

 It possesses the greatest sensibility, its results are most 

 reproducible ; by its use, things measured equal to the same 

 thing measure equal to each other, and the sum of the 

 measurements of the parts equals the measurement of the 

 whole. 



Illumination. — An illumination of 25 metre candles was 

 chosen since, by the means described below, this was found 

 to correspond closely to the high illumination at which the 

 results of the flicker and equality of brightness methods 

 become the same. This is a practicable illumination for the 

 usual laboratory light sources and photometers ; it corre- 

 sponds to a comfortable working illumination for reading, 

 &c, and the changes in visual colour sensitiveness on going 

 to the still higher illuminations common by daylight are 

 probably small, perhaps negligibly so. 



Size of Photometric Field. — A photometric field of 2° 

 diameter was selected ; that is, the smallest field used in 

 the previous work, for the reason that the changes in the 

 luminosity curve due to changes in illumination are least 

 with the small field. This field size, together with the high 

 order of illumination chosen, would permit considerable 

 latitude in the illumination without appreciable effect on 

 the result. 



Bright Surrounding Field. — The small photometric field 

 was surrounded by a bright field of about 25° diameter, 

 maintained at approximately % the same brightness as the 

 photometric field. This bright area, although previously 

 not found to affect the results of the flicker photometer to 

 an extent greater than the errors of measurement, was used 

 for the greater comfort which it gives when observing with 

 a small field. It is not an essential part of the scheme of 

 measurement, but for the sake of complete uniformity should 

 be used always, if at all. 



Apparatus. 



The apparatus was the same as in the previous work. 

 A Hilger spectrometer with a | x2 mm. slit in place of 

 the eyepiece ; the light from the comparison lamp (4 watt 

 carbon), after falling upon a magnesium oxide surface, is 

 reflected into the spectrometer telescope from a first surface 

 platinized glass mirror arranged to oscillate and so form a 

 flicker photometer ; the diaphragm which limits the size of 



